1. Field of the Invention
The present invention relates to a printing apparatus and control method therefor.
2. Description of the Related Art
There is known a printing apparatus which prints information such as text or images on a printing medium. The printing method is, for example, an inkjet printing method of printing with ink. A discharge energy generation element used to discharge ink is, for example, a heat generation element such as a heater, or a piezoelectric element. Any method using such a discharge energy generation element controls discharge of an ink droplet based on an electrical signal.
According to a method using the heat generation element, a voltage is applied to the heat generation element, instantaneously boiling ink near it. An ink droplet is discharged by an abrupt blowing pressure generated by a phase change of ink upon boiling (Japanese Patent Laid-Open No. 2006-175744). In contrast, according to a method using the piezoelectric element, a voltage is applied to the piezoelectric element to displace it. An ink droplet is discharged by a pressure generated upon displacement.
When printing on a low-quality printing medium such as plain paper using a printhead adopting either method described above, so-called low-pass printing is executed to complete printing on the same printing area on the printing medium in, for example, one to four scan operations. When printing on a high-quality printing medium such as a photo medium, so-called multi-pass printing is done to complete printing on the same printing area on the printing medium in, for example, four to 24 scan operations. The multi-pass method can reduce printing variations and improve print quality.
Recently, higher speed and higher quality print processing is being pursued in order to meet an increasing demand for photo printing. High-speed printing, that is, low-pass printing is required even for high-quality media.
When print processing is performed quickly for a high-quality medium using low-pass printing, print quality is satisfactory with even numbers of passes such as four, six, or eight; however, results are poor on odd numbers of passes such as one, three, or five. More specifically, band-like (to be also referred to as a band) nonuniformity is generated.
This mechanism will be explained. Ink discharged from each nozzle array of the printhead has a velocity component accompanying scanning of the printhead. In addition, the ink is affected by an air flow (called a Couette flow) generated when the head surface is moved along with the scanning of the printhead. Discharged ink including a main droplet and satellite flies in the printhead scanning direction.
The printhead is scanned within the apparatus, that is, a closed space. An air flow swept away by the scanning of the printhead is reflected by the wall surface of the apparatus and enters the interval between the printhead and the printing medium as an incoming air flow. The incoming air flow is generally asymmetrical in volume between forward printing and backward printing. This is because the internal structure of the printing apparatus and the structure of the printhead are horizontally asymmetrical in the printhead scanning direction. Due to the incoming air flow, the distance between the main droplet and the satellite (to be also referred to as a main-satellite distance) differs between forward printing and backward printing.
FIG. 11A shows a case in which an incoming air flow generated in backward printing is larger than that generated in forward printing. In this case, the main droplet and satellite are swept back in a direction opposite to the scanning direction much more in backward printing than in forward printing. In particular, small volume satellites are largely swept back in the direction opposite to the scanning direction. As a result, in reciprocal scanning, the main-satellite distance differs between nozzle arrays on the upstream side (windward) in the printhead moving direction (scanning direction), or those on the downstream side (leeward) in the printhead moving direction.
From a comparison between windward and leeward nozzle arrays in forward printing and backward printing, the main-satellite distance of the windward nozzle array is shorter in the two cases. This is because an incoming air flow blowing into the leeward nozzle array is much more attenuated than that blowing into the windward nozzle array, and the distance by which the main droplet and satellite are swept back in the direction opposite to the scanning direction decreases.
A flow velocity distribution shown in, for example, FIG. 11B affects ink discharged from each nozzle array of the printhead. As shown in FIG. 11B, a Couette flow generated in the printhead scanning direction is swept back by an incoming air flow in the direction opposite to the scanning direction. Flow velocity distributions different between the forward and backward directions and between the windward and leeward nozzle arrays act on inks discharged from the respective nozzle arrays of the printhead.
More specifically, the incoming air flow pushes the Couette flow most for the R array positioned windward in backward printing. The flying distance of a main droplet and satellite discharged from the R array in the printhead scanning direction becomes relatively small. In contrast, the incoming air flow is attenuated for the R array positioned leeward in forward printing. Thus, the distance by which the main droplet and satellite are swept back by the incoming air flow becomes very short, and only the Couette flow acts substantially. As a result, a main droplet and satellite discharged from the R array positioned leeward in forward printing fly by a relatively large distance in the printhead scanning direction.
FIG. 11C shows parts of the L and R nozzle arrays of the printhead and the image of bands adjacent in the printing medium conveyance direction upon printing with these nozzle arrays. When the printhead having this arrangement executes reciprocal scanning using an odd number of passes (e.g., 3-pass reciprocal printing), a band formed by “forward 1→backward 1→forward 2 (numerals after “forward” and “backward” indicate turns of scanning)” and one formed by “backward 1→forward 2→backward 2” are adjacent to each other. More specifically, these bands adjacent to each other in the printing medium conveyance direction are different in pass in forward printing and backward printing. The difference in pass results in the above-mentioned nonuniformity. This is because, in between bands formed by odd number of passes, difference in how the area factors are filled by main droplets and satellites is generated. The difference in the number of gaps leads to a difference in density between adjacent bands, generating band nonuniformity.